Markus Kalberer, one of the scientists involved in the study from the centre for atmospheric science at the University of Cambridge, said: "We showed that, in contrast to the general belief, scooters can be a dominant source of air pollution, including soot and organic particles, in urban areas.

"We showed that the particles transformed the atmosphere from these scooter emissions are especially toxic."

Scooters typically use two-stroke engines, which are far less efficient at burning the fuel they use.

The exhaust systems are also less effective at removing pollutants than catalytic converters found in most modern cars.

This means that exhaust fumes from scooters contain higher levels of the chemicals found in the fuel and oil they use.

The scientists measured the emissions produced by range of different vehicles and found that two stroke engined scooters emit significantly more primary organic aerosols and volatile organic compounds from incomplete burning of the fuel.

They also produced significant secondary organic aerosols as gases released from the exhaust reacted with the air and microscopic particles of soot, which are known to increase the risk of heart and lung disease.

They also found that older two stroke vehicles produced reactive oxygen molecules that are known to be potentially harmful lung health.

As well as testing different scooters in the laboratory, the scientists assessed the air pollution in cities in Europe, the US and Asia.

&lt;noframes&gt;Datatable: Pollution produced by vehicle type&lt;/noframes&gt;

They claim that reducing the number of two-stroke scooters would improve urban air quality significantly and cost-effectively, even though less than 1 per cent of fuel in the European Union is currently used in scooters.

Several Chinese cities have banned scooters and report markedly reduced air pollution, even when other traffic has increased.

In a paper published in the journal Nature Communications, the authors said that in some cities scooters may contribute up to two thirds of the roadside air pollution.

They said: "Using average two stroke scooter emission factors... suggests that two stroke scooters contribute to around 60 per cent of roadside POA in Bangkok, where they account for 10 per cent of fuel consumption.

"Our data suggest that two stroke scooters are a signiﬁcant, and in many cities, the largest source of vehicular particular matter and toxic secondary organic aerosols and aromatic hydrocarbons, despite being a relatively small fraction of the total ﬂeet.

"Therefore, given the alternative technologies available, restrictions on two stroke scooters, already implemented in China, could improve air quality in many cities around the globe."

Stephen Platt, the lead author of the paper from the Paul Scherrer Institute in Switzerland, added: "Cars and trucks, particularly diesel vehicles, are thought to be the main vehicular pollution sources.

"This needs re-thinking, as we show that elevated particulate matter levels can be a consequence of ‘asymmetric pollution’ from two-stroke scooters, vehicles that constitute a small fraction of the ﬂeet, but can dominate urban vehicular pollution through organic aerosol and aromatic emission factors up to thousands of times higher than from other vehicle classes."